UI circuits are systems of UI items (for example, widgets). They are similar to ordinary
circuits as provided by FRP.Grapefruit.Circuit but have the additional feature of providing
parts of user interfaces.

an action running the circuit and returning the value of the output signal’s
first occurence

Runs a user interface circuit.

run quits when the output signal of the circuit has a first occurence. The universal
quantification of the circuit’s era parameter ensures that the circuit does not use
signals which are produced outside the circuit and therefore avoids era mismatches.